Dragline control



W. R. HARDING ETAL DRAG LINE CONTROL Filed May 8, 1945 2 Sheets-Sheet 2 Fl 94. E

b 3 400 4900 -$ao s50 I 2400 31'50 load/7m (75!{1/2) l l I l l 1 1 I l l I I I l i l 200- l I z l l i I l l I l I i 1 v V i l 806 moo Z400 4M4 1707 5. (foryue) --400 9 f WITNESSES: INVENTORS 141M M M William K. Harding and Laurence P ATTORNEY Patented July 31, 1945 DBAGLINE CONTROL William R. Harding, Export, Pa., and Iawrence G.

Opel, Hobe Sound, Fla., asslgnors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 8, 1943, Serial No. 486,178

14 Claims.

This invention relates to drag line controls and more particularly to variable voltage drives for operating the walker of such a machine.

The walker drive of drag-lines is subjected to operating conditions of extreme severity. The torque to be developed by the drive motor varies during a walking cycle from a high maximum to zero and assumes temporarily negative values. The maximum torque occurs at the instant of lifting. From then on the torque decreases to zero as the machine is lifted until the point is reached whence the machine starts moving downward under its own weight. During the downward motion the torque becomes negative. That is, the torque is now generative, and increases in its negative value until the machine sits down at the end of the walking cycle. The torque at sitdown is lower than that at pickup due to the difference in static and moving friction in the gearings of the walker.

These heavy and extremely varying loads are apt to cause unsatisfactory operating conditions and stresses in the structure, gearing and other equipment of these machines. For instance, due to the fact that the load passes to high negative values at the end of the walking cycle of a drag line as referred to above, the sit-down of the machine tends to occur at relatively high speed thus tending to stress the machine by impact on the ground.

In order to reduce these difliculties, contactors have been employed to regulate the current supplied to the drive motor from a voltage controlling generator. Such contactors, however, entail other drawbacks since they have to carry high current intensities, such as 2000to 3000 amperes on larger machines. This method has also the disadvantage that it does not permit a satisfactory reduction in the sit-down speed of the walker.

It is an object of our invention to improve dragline and other digging on hoisting machines or the type here in point by reducing or eliminating the just-mentioned drawbacks of the extreme load conditions to be encountered.

Another somewhat more specific object is to provide a walking drive which ensures a walking cycle in which the speed of the motor is high during the lifting and idle periods of the walker and is low during the lowering period, the torque being at a maximum at the beginning of the lifting operation.

It is also an object of our invention, to provide a drag-line shovel with a walker drive that ensures maximum torque and low initial speed at the lifting moment in coniunctlon with a high negative torque and low speed at the moment of sit-down while causing the walking speed to increase from zero to a maximum and thence to decrease gradually back to zero when passing through a complete walking cycle, the walking speed being understood as the tangential speed of the tub. In other words, the invention, in this aspect, aims at providing most favorable walkinggo terrupting devices in the regulating network proper in order to eliminate the drawbacks inherent in the use of such contact devices.

These and other objects and advantages are achieved, according to the invention, by a particular variable voltage drive which will be described hereinafter with reference to the embodiments shown in the drawings in which:

Figure 1 is a diagrammatic side elevation of a drag-line shovel having part of its cab wall broken away to show the gearing and motor 'of the walker drive;

Figs. 2 and 3 illustrate the circuit diagrams of two different drive control systems to be used for operating the walker drive motor of a shovel as represented by Fig. 1, while Figs. 4 and 5 show explanatory curves serving to elucidate the operation of drive systems as shown in Figs. 2 and 3.

Referring to Fig. 1, numeral i denotes the cab of the drag-line shovel. The cab is mounted on a supporting frame structure I which is rotatable on a circular rail 8. The rail is carried by a tub I of cylindrical shape. A boom 5 is pivoted at 6 to the frame structure 2 and can be adjusted in its angular position by means of a boom line I. This line is guided by one of a set of rollers 8 into the interior of the cab I where it is wound on a power-operated drum (not apparent in the illustration), The set of rollers l is Journaled in a superstructure 3 connected with the supporting frame structure I. As a rule, the position of the boom is not changed during a diss s operation The bucket of the shovel is denoted by II. The bucket is suspended from a hoist line H which passes over a roller II at the upper end of the boom I to another roller of set I and thenc into the cab I where it is wound on another power-operated drum (not visible in the lilustration). By operating the latter drum, the bucket II is hoisted or lowered in accordance with the desired shoveling operation.

A drag line I: is also attached to the bucke II and passes over a guiding roller II into the cab I also to be wound up on a power-operated dragging drum (not shown). The line I2 is controlled to drag the bucket II toward the cab I when performing the digging operation.

In order to move the shovel along the ground, a walking mechanism hereinafter called briefly a "walker" is provided. This walker contains two walking pontoons II arranged at opposite sides of the cab and frame structure. Each pontoon I4 is attached to a walking beam II which is Journaled to a crank arm Ii mounted on a shaft II. The crank arms It at the two sides of the machine have the same angular position relative to their common operating shaft I1, A transmission gear II of suitable construction is provided for connecting the shaft II. with a drive motor M. i

The drive motor of the walker may consist of a motor separate from those operating the drums of one on several of the lines 1, II and I2. However, it is also possible, and as a rule preferable, to employ a single motor for operating all of the above-mentioned machine portions, a selective coupling and gearing being used for connecting the motor with the drum or shaft to be operated at a time.

During a w lking cycle the motor M rotates shaft I1 and crank arm II in the counterclockwise direction. As a result, the walking beam II and the pontoons II are moved towards the lefthand side of the illustration until the pontoons reach ground in the position indicated by the broken line II. From then on, while the motor continues to rotate the crank arm II, the pontoons exert pressure against the ground and cause the tub I and hence the entire machine to be lifted in the direction of the arrow 22, the lifting motion beginning at the rear end of the machine. At the same time, the pontoons impart a horizontal motion to the machine towards the rear as is indicated by the arrow II. As a result, the front end of the machine is dragged over the ground. In order to ensure this operation, the shaft I1 is located relative to the center of gravity, at II, of the entire machine so as to lie below and in the rear of this center. At the end of a walking cycle the pontoon I4 is lifted off the ground. If the motor M is now stopped, the pontoons are idly suspended from their respective crank arms. If a further motion of the machine is necessary, the walking operation is continued to perform one or more additional cycles Due to the cylindrical shape of the tub I and the possibility of rotating the cab and frame structure relative to the tub, the machine can be moved in any desired direction by placing the supporting structure and the parts attached thereto into a corresponding angular direction with respect to the tub I.

The invention proper is concerned with the electrical energizing and control system connected to the motor M and serving to govern the torque and specific conditions of this motor and of the walker operated thereby. An electrical system according to the invention is exemplified by Fig. 2.

According to the control system of Fig. 2, the motorlfhasanarmature III andaseparateiy ontrolled ileld winding I I2. This field winding .1; connected through leads III and III and adjusting rheostat- III to a constant voltage exclter generator E.

The armature III of the motor M is series-connected, as shown, to the armature I II of a control generator G. The generator G has a diiferential series field Winding I I4 and interpole and compensating windings Ill all connected in series with the armatures III and III. The genetator G is also provided with a separately e1- cited winding H2 and a further field winding III.

. III permits adjusting the separate The field winding I I2 is connected through a suitable rheostat III to the exciter E. The rheostat excitation of the generator G to a desired value.

The field winding III is connected by a regulator circuit III with the armature III of a regulating generator R. The regulator circuit III includes an adjusting rheostat I in series connection to the generator field winding III.

The regulator R has a principal control field winding I21 and a self-sustaining shunt ileld winding I having a calibrating rheostat I". Winding I is connected in parallel to field III and its rheostat III. The principal field I23 is connected through a circuit I20 across the interpole and compensating field windings III of the generator G, a rheostat I" being provided to calibrate the energization of field winding III. When in operation, the field winding III is excited in dependence upon the current flowing through the generator field windings III and consequently inaccordance with the current supplied from the generator G to the motor M.

The exciter E has its armature III acted upon by a series-connected field winding III and by a self-excited shunt winding I. An adjusting rheostat III is connected in the circuit of the exciter shunt field winding I in order to select a proper output voltage of the exciter.

The armatures III, III and III are driven at constant speed. While this can be effected with the aid of separate motors, the embodiment according to Fig. 2 is so designed that these three armatures are mounted on a common shaft denoted by Ill. The armature Ill of a constant speed motor CSM is also mounted on this shaft and serves to rotate it at constant speed. An alternating current induction motor such as a squirrel cage motor may be used for this purpose.

Due to the constant speed of the exciter armature I SI, the output voltage of the exciter remains constant during the walking operation; that is. once the rheostat I It has been properly adjusted, its adjustment is not changed during the control operation proper. For the same reason, and assuming a constant adjustment of rheostat I II, the excitation of the motor field winding Ill remains constant during the walking operation. As a result, the speed and torque characteristic of the walker motor M are dependent only on the voltage and current in the feed circuit III! and hence on the control operation on the generator Cl.

The series-connected field winding II of the generator G and the shunt type iield III of the same generator act in opposition to each other with respect to their control function on the generator armature III. This is indicated by the arrows marked A and B. The differential eifect is adjusted to obtain the before-mentioned desired walking characteristic as will be more clearly apparent from the following explanation.

The field component produced by the separately excited field Winding H2 is not affected by the above-mentioned differential operation and hence determines the basic voltage and current characteristic in the feed circuit I00.

This characteristic can be controlled by means of the rheostat I I6. It will be understood that while we have shown a slide contact rheostat in the circuit of the field winding H2, one of the customary types of contact controllers may be used instead. Likewise, the other rheostats may be replaced by functionally equivalent circuit means of other type and design without affecting the function and advantages of the control system.

Since the energization of the shunt-type winding H3 of the generator G is dependent on the output current of the regulator armature I2I and also on the adjustment of the rheo-stats I46 and I66, it is apparent that the feed current in circuit I depends essentially on the operation and adjustment of the just-mentioned elements. The excitation of the generator winding H3 varies in dependence upon the load current in the feed circuit I00 due to the fact that the principal field winding I23 of the regulator R is excited in accordance with the feed current. The basic values of the excitation supplied by the regulator armature I2I to the generator field winding H3 can be adjusted by varying the effective resistance of rheostat I46 in regulator circuit H0 and by varying the field resistance of the self-sustained field winding I24 with the aid of rheostat I66. By the latter two adjustments the desired regulation of the walker operation can be obtained while the separate field winding II2 of the generator G is excited so as to give the machine the necessary torque to lift and to sit down.

Before discussing the control function of the above-described system in further detail, it appears appropriate to first indicate some modifications which can be made without departing from the principle and advantages of the invention.

Referring to the self-sustaining exciter field I24, it will be understood that this field may also be designed as a series field. Furthermore, the invention is not necessarily predicated upon the connection of the principal regulator field across the interpole and compensating windings of the main generator. Since the requirement of energizing the principal regulator field in accordance with the load current in the feed circuit can also be obtained by other circuit connections, any such connection capable of ascertaining the desired dependency can be chosen instead of the one illus trated in Fig. 2. For instance, a potentiometric resistor may be arranged in the feed circuit, and the voltage for energizing the principal regulator winding can be taken from this resistor. Furthermore, a self-sustaining field winding as aforementioned, though affording special advantages, is not necessary in all cases.

In order to elucidate some of the just-mentioned possibilities, reference is had to the control system illustrated in Fig. 3. This system contains the same main elements as that of Fig. 2, namely, a variable speed direct current motor M for operating the walker of a drag-line shovel, a main generator G for controlling the motor, and a regulating generator R for controlling the generator G, an exciter E as a convenient source ofconstant voltage, and a constant speed motor CMS for driving the armatures of generator G, regulator R and exciter E. Also as regards any other details, the system according to Fig. 3 is similar to the corresponding elements shown in Fig. 2. In order to facilitate a comparison and for simplifying the following description of Fig. 3, the last two digits of the reference numerals used in Fig. 3 are identical with the corresponding digits of the numerals in Fig. 2 wherever similar or functionally comparable circuit elements are concerned.

The armature 20I of motor M according to Fig. 3 is acted upon by a separately excited field winding 202 which is energized, through a suitable rheostat 206, from the exciter E. Numeral 23I designates the armature of the exciter and 233 the exciter field winding.

The armature 20I of motor M is series-connected with a potentiometer resistor 256 in a feed circuit 200 energized from the armature 2H of the main generator G. A self-excited field winding 2| 4 of the generator is connected in series with the armature 2] I. A second generator field winding 2 I 3 acting in opposition to winding -2I4 is series-connectedwith a rheostat 246 in a regulator circuit 2l0 energized from the armature 22I of the regulating generator R whose field winding 223 is connected through leads 220 across the rheostat 256. The generator G has a separately excited field winding 2I2 energized, through rheostat 2| 6, from the exciter E. The armatures 2H, HI and 23I are mounted on a common shaft.240 which is driven by the constant-speed motor CSM.

Sinc the system according to Fig. 3 has its regulating generator R controlled in accordance with the current fed from the generator G to the motor M, and since the system provides also the possibility of regulating th current and voltage in the regulator circuit 2l0 by varying the resistance of the rheostats at 246 and 2I6, the basic function of this system is similar to the above-described operation of the system shown in Fig. 2. It will be noted, however, that the system of Fig. 3 does not include a self-sustaining field winding on the regulator R corresponding to the field winding I24 of the system previously described.

The control operation of the above-mentioned systems and their effect on the walking operation of the drag-line will b more fully understood from the load-voltage or torque-speed characteristics shown in Figs. 4 and 5.

Referring to Fig. 4, the abscissa of th coordinate system shown in this illustration indicates the load current in amperes fed from the generator G to the motor M while the ordinate represents the voltage of this current. Consequently, the curves a and b, or a and b, or a" and b" represent voltage-load characteristics of the walker drive. Since the speed of the walker drive motor is proportional to the energizing voltage because the separately energized motor field is kept constant during the walking operation, the ordinate of the diagram in Fig. 1 represents also the speed of the walker drive. For the same reason the values indicated on the abscissa of the diagram represent also the torque exerted by the drive motor on the walking mechanism since under the above-mentioned conditions the torque is proportional to the load current.

Curves a and 22 pass through the zero point 0 of the coordinate system and are obtained when not applying excitation to the separately excited field winding H2 or 2I2 of the generator G. Curve b is obtained with more resistance (at I46 or 246) in the armature circuit (H0 or 2l0) of the regulator R. When excitation is applied to the separately excited field winding of the generator G, the corresponding curves ar shifted upwardly, for instance, the curves a and b passing through point P are obtained with an excitation on the separately excited field winding of the generator that corresponds to a generator voltage of 125 volts. Similarly, the curves :1 and I) both passing through point S correspond to a separate excitation of the generator equivalent to 250 volts in the generator armature circuit. The curves b and b" result from the use of a higher resistance in the regulator circuit than in the case of the respective curves 0. and a".

The family of curves represented in Fig. 4 refer to a control system in which no self-sustaining field is applied to th regulator R as is illustrated in Fig. 3 or obtained in Fig. 2 when omitting r disconnecting the field winding I24.

In order to render the control system properly operating, the resistance of the regulator circuit and the value of the separate generator excitation should be chosen so that the operating characteristic lies above the zero axis within the entire rang of the walking operation. Curves b, a", and I)", possibly also curve a, satisfy this condition.

Let us now assume that 125 volts from the separate generator field (H2 or 2i2) are sufficient under given conditions to produce the torque necessary to lift the machine when operating the walker and that the value of this torque is developed at about 2000 amperes. Then the operation of the walker might correspond to curve b. When starting the operation of the walker, the speed of the motor increases from the value P to a maximum value and decreases along the right-hand portion of curve I) to the value Q at the maximum torque suiiicient for lifting the machine. Thence the load decreases at increasing voltage. That is, th operating point now travels along curve I) from point Q towards the left-hand side of the diagram. After passing through the maximum, the speed decreases until, for instance, at point R, i. e. at about 1300 amperes negative, the machine sits down lightly. If, when the torque becomes negative, the generator voltage is sufficiently reduced, the walker will be stopped in mid-air. The speed and torque values effective in the walker change proportionately to the above-explained changes in voltage and load.

The family of curves shown in Fig. 5 refer to a walker controlled system in which the regulating generator R is provided with a self-sustaining field such as that produced by the field winding I24 shown in Fig. 2. The curves 0, c, d, d, e, I, g, and h correspond to different resistances in the regulator circuit and in the circuit of the self-sustaining winding of the regulator. For instance, curves 0 and c refer both to a regulating condition in which the resistance of the self-sustaining field circuit is the same while the resistance in the regulator circuit is larger in the case of curve 0' than for curve 0. Curves d and d, refer to the same resistance in the self-sustaining field circuit, this resistance being lower than in the case of curves 0 and c. The curve d refers to a larger resistance in the regulator circuit than curve d. Curves e and 1 refer to adjustments in which the resistance of the regulator circuit is approximately the same while the resistance of the self-sustaining regulator field is larger for curve e than for curve I. Still other resistance combinations are referred to by curves 0 and h.

All of the curves shown in the diagram of Fig. 5 refer to zero excitation at the separately excited field winding (H2 or N2) of the generator G. In actual operation, enough excitation must be applied to the separately excited winding to raise the operating characteristic above zero. The effect of such an excitation is equivalent to shifting the load current axis in Fig. 5 downwardly by the amount of generator output voltage which corresponds to the excitation of the separate generator field.

The dotted part of curves 1', a and h is indeterminable, that is, a reversal takes part in the approximate location of the dotted parts but this reversal is in reality not desultory as shown by the dotted lines for simplicitys sake, but involves to a certain extent a gradual transition depending upon the impedance values of the circuits involved. However, the exact course in this portion of the curves is not essential for an understanding of the invention, since regardless of the exact shape or this curve portion, the load and voltage, and accordingly the torque and speed characteristics, have the same basic behavior during a walking cycle as the curves previously discussed with reference to Fig. 4. It will also be seen from a comparison of Fig. 5 with Fig. 4 that the provision of a self-sustaining field winding in the regulating generator affords a considerably greater range of adjustment than a system without such a winding.

The foregoing shows that the desired walking characteristic aimed at by our invention and mentioned in a preceding portion of this spectflcation is, in fact, fully obtainable by a walker drive system in accordance with the principles of this invention. It is also apparent from the foregoing that such a system does not have the tendency to increase the voltage and speed when the load current becomes negative, a highly disadvantageous operating condition to be coped with in known shovel controls. These advantages are achieved without the use of contactors and other circuit interrupting devices in the control network proper, so that the disadvantages inherent in such contactors for high load currents are also avoided.

While our invention as described in the foregoing refers to walker drives of drag lines, it will be understood that its principle and means are also applicable to walking equipment if used in connection with other than drag lines and that the invention can be used to advantage wherever a hoisting machine or shovel operates in accordance with a speed and torque cycle which involves load variations similar to those discussed in the foregoing. Hence, it should be understood that the system described in this specification serves the purpose of disclosure and exemplification but is not intended to limit the invention to the illustrated-embodiment.

We claim as our invention:

1. An electric drive for operating walker-type propulsion mechanisms, comprising a separately excited drive motor of variable speed, a main generator having an armature circuit connected to said motor to supply controllable voltage thereto, said main generator having a self-excited field winding connected with said armature circuit and a separately excited field winding acting in opposition to said self-excited field winding, a regulating generator having a regulator armature and two regulator field windings, said regulator armature being connected with said separately excited field winding to energize the latter, one of said regulator field windings being connected with said armature circuit to be energized in dependence upon the load current of said motor, said other regulator field winding being connected to said regulator armature to provide self-excitation, control means interposed between said latter regulator field winding and said regulator armature for adjusting said self-excitation, further control means arranged between said regulator armature and said separately excited field winding for controlling the energization of said latter winding and thereby the energizat on of said motor, and means for driving said two armatures at substantially constant speed.

2. An electric drive for walking mechanisms, comprising a separately excited drive motor, a. main generator having an armature circuit connected to said motor to supply controllable voltage thereto, said main generator having a control field winding and two differential field windings acting in opposition to each other, a source of constant voltage connected to said control field winding, adjustable circuit means arranged between said source and said control field winding for controlling the energization of the latter, one of said difierential field windings being series connected in said armature circuit, a regulating generator having a regulator armature connected to said other differential field winding and having a regulator field winding connected with said armature circuit to be energized in dependence upon the load current of said motor, variable circuit means arranged between said regulator armature and said separately excited field winding for controlling the energization of said latter winding and thereby the energization of said motor, and means for driving said two armatures at substantially constant speed.

3. An electric drive for walking mechanisms, comprising a separately excited drive motor of variable speed, a main generator having an armature circuit connected to said motor to supply controllable voltage thereto, said main generator having a control field winding and two differential field windings acting in opposition to each other, a source of constant voltage connected to said control field winding, adjustable circuit means arranged between said source and said control field winding for controlling the energization of said latter winding, one of said difierential field windings being connected with said armature circuit, a regulating generator having a regulator armature and two regulator field windings, a regulator circuit connecting said regulator armature with said other differential field winding to energize the latter, one of sa d regulator field windings being connected with said armature circuit to be energized in dependence upon the load current of said motor, said other regulator field winding being connected to said regulator circuit to provide self-excitation, circuit control means interposed between said latter regulator field winding and said regulator circuit for adjusting said self-excitation, variable circuit means connected with said separately excited field winding for controlling its energization and thereby the energization of said motor, and means for driving said two armatures at substantially constant speed.

4. An electric drive for walking mechanisms, comprising a separately excited drive motor, a main generator having an armature circuit connected to said motor to supply controllable voltage thereto, said main generator having a separately excited field winding, a self-excited field winding, and interpole and compensating windings, said separately excited field winding and said self-excited field winding being arranged differentially relative to each other, said selfexcited field winding and said interpole and compensating windings being connected with said armature circuit, a regulating generator having a regulator armature connected to said separately excited field winding to energize the latter and a regulatory field winding connected across said interpole and compensating winding so as to be energized in accordance with the load current of said motor, variable circuit means arranged between said regulator armature and said separately excited field winding for controlling the energization of said latter winding and thereby the energization of said motor, and means for driving said two armatures at substantiallyconstant speed.

5. An electric drive for Walking mechanisms, comprising a separately excited drive motor, a main generator having an armature circuit connected to said motor to supply controllable voltage thereto, said main generator having a separately excited field winding, a self-excited field winding, and interpole and compensating windings, said separately excited field winding and said self-excited field winding being arranged diiierentially relative to each other, said self-excited field winding and said interpole and compensating windings being series connected with said armature circuit, a regulating generator having a regulator armature and two regulator field windings, a regulator circuit connecting said regulator armature with said separately excited field winding to energize the latter, one of said regulator field windings being connected with said armature circuit to be energized in dependence upon the load current of said motor, said other regulator field winding being connected to said regulator circuit to provide self-excitation, circuit control means interposed between said latter regulator field winding and said regulator circuit for adjusting said self-excitation, variable circuit means arranged between said regulator armature and said separately excited field winding for controlling the energization of said latter winding and thereby the energization of said motor, and means for driving said two armatures at substantially constant speed.

6. A variable voltage drive for operating a walker, comprising a drive motor for operating.

said walker, a main generator having an armature circuit connected to said motor to supply controllable voltage thereto, said main generator having a control field winding and two differential field windings acting in opposition to each other, a source of constant voltage and adjustable circuit means connected to said control field winding for controlling the energization of said latter winding, one of said differential field windings being connected with said armature circuit, a regulating generator having a self-excited field wind ing and a regulator armature connected to said other diiierential field winding to energize the latter and a regulator field winding connected with said armature circuit to be energized in de,- pendence upon the load current of said motor, and means for driving said two armatures at substantially constant speed.

7. A variable voltage drive for walkers, comprising a direct-current drive motor having a motor armature and a separately excited motor field winding, a main generator having an armature and two field windings operative in opposition to each other, a feed circuit connecting said generator armature with said motor armature to supply the latter with controllable voltage, one

oi said generator field windings being connected to said feed circuit to provide self-excitation, a regulating generator having a regulator armature connected to said other main generator field winding to supply variable energization thereto, said regulating generator having a regulator field winding connected to said feed circuit so as to be energized in accordance with the load current of said motor armature and including a self-energizing winding connected to said regulator armature, variable circuit means arranged between said regulator armature and said other generator field winding for controlling the energization of said latter winding, an exciter generator having an exciter armature electrically connected with said motor field winding to provide it with constant energizing voltage, a common shaft connecting said main generator armature, regulator armature and exciter armature, and means for driving said shaft at substantially constant speed.

8. An electric drive for a propulsion mechanism having a walker, comprising a direct-current drive motor and having a motor armature and a separately excited motor field winding, a main generator having an armature and three field windings of which two are differential relative to each other, a feed circuit connecting said generator armature with said motor armature to supply controllable voltage to the latter, one of said diiferential generator field windings being connected to said feed circuit to provide self-excitation. a regulating generator having a regulator armature connected to said other difi'erential field winding to supply variable energization thereto and being provided with a self-exciting field winding connected to said regulator armature, said regulating generator having a regulator field winding connected to said feed circuit to be enersized in accordance with the load current of said motor, an exciter generator having an exciter armature electrically connected with said motor field winding and with said remaining generator field winding to supply constant energizing voltage thereto, variable circuit means disposed between said exciter armature and said latter generator field winding for regulating the voltage of said feed circuit, and means for driving said three generator armatures at constant speed.

9. A drag line provided with walking pontoons, a crank shaft for cyclically operating said pontoons, a direct-current motor geared to said shaft, a main generator having an armature circuit connected to said motor for energizing said motor, said'maln generator having a self-excited field winding and a separately excited field winding disposed differentially relative to each other, said self-excited winding being connected with said armature circuit, a regulating generator having a regulator armature connected to said separately excited field winding to energize the latter and a regulator field winding connected with said armature circuit to be energized in dependence upon the load current of said motor, variable circuit means arranged between said regulator armature and said separately excited field winding for controlling the energization of said latter winding and thereby the energization of said motor, and means for driving said two armatures at constant speed, whereby the torque imparted by said motor through said shaft to said pontoons is at a maximum at the instant of zero speed when lifting the drag line and decreases to negative values before returning to zero when the drag line sits down again at the end of a walking cycle.

10. A drag line provided with walking pontoons, a crank shaft for cyclically operating said pontoons, a direct-current motor geared to said shaft, a main generator having an armature circuit connected to said motor for energizing said motor, said main generator having a self-excited field winding and a separately excitedfield windings disposed diflerentially relative to each other, said self-excited winding being connected with said armature circuit, a regulating generator having a regulator armature and two regulator field windings, a regulator circuit connecting said regulator armature with said separately excited field winding to energize the latter, one of said regulator field windings being connected with said armature circuit to be energized in dependence upon the load current of said motor, said other regulator field winding being connected to said regulator circuit to provide self-excitation, circuit control means interposed between said latter regulator field winding and said regulator circuit for adjusting said self-excitation, variable circuit means arranged between said regulator armature and said separately excited field winding for controlling the energization of said latter winding and thereby the energization of said motor, and means for driving said two armatures at constant speed, whereby the torque imparted by said motor through said shaft to said pontoons is at a maximum at the instant of zero speed when lifting the drag line and then decreases to negative values before returning to zero when the drag line sits down again at the end oi a walking cycle.

11. A drag line provided with walking pontoons, a crank shaft for cyclically operating said pontoons, a direct-current motor geared to said shaft, a main generator having an armature circuit connected to said motor for energizing said motor, said main generator having a control field winding and two differential field windings acting in opposition to each other, a source of constant voltage connected to said control field Winding, regulating circuit means arranged between said source and said control field winding for controlling the energization of said latter winding, one of said differential field windings being connected with said armature circuit, a regulating generator having a regulator armature connected to said separately excited field winding to energize the latter and a regulator field winding connected with said armature circuit to be energized in dependence upon th load current of said motor, variable circuit means arranged between said regulator armature and said separately excited field winding for controlling the energization of said latter winding and thereby the energization of said motor, and means for driving said two armatures at constant speed, whereby the torque imparted by said motor through said shaft to said pontoons is at a maximum at the instant of zero speed when lifting the drag line and then decreases to negative values before returning to zero when the drag line sits down again at the end of a walking cycle,

12. A hoisting machine having a direct-current hoist motor, a main generator having an armature circuit connected to said motor, said main generator having a self-excited field winding and a separately excited field winding disposed diiferentially relative to each other, said self-excited winding being connected with said armature circuit, a regulating generator having a regulator armature and two regulator field windings, a regulator circuit connecting said regulator armature with said separately excited field winding to energize the latter, one of said regulator field windings being connected with saidarmature cir, cult to be energized in dependence upon the load current of said motor, said other regulator field winding being connected to said regulator circuit to provide self-excitation, circuit control means interposed between said latter regulator field winding and said regulator circuit for adjusting said self-excitation, variable circuit means arranged between said regulator armature and said separately excited field winding for controllin the energization of said latter winding and thereby the energization of said motor, and means for driving said two armatures at constant speed, whereby the torque 01 said motor is at a maximum at the instant of zero speed when hoisting and decreases through zero to negative torque values when lowering thence returning to zero at the instant of setting at the end of a hoisting cycle.

13. A variable voltage drive for operating walker-type propulsion mechanisms, comprising a separately excited drive motor of variable speed, a main generator having an armature circuit connected to said motor to supply controllable voltage thereto, said main generator having a field winding series-connected in said armature cir cult and a separately excited field winding acting in opposition to said self-excited winding, a regu lating generator having an armature connected with said separately excited field winding for energizing the latter and having a regulating field winding connected with said armature circuit so as to be energized in dependence upon the load current of said motor, operator-controllable impedance means arranged between the armature of said regulating generator and said separately excited field winding for controlling the energization of said latter winding and thereby the energization of said motor, and means for driving said armature of said main generator and said regulating generator at substantially constant speed. 14:. A variable voltage drive, comprising a direct current drive motor, a first generator having an armature circuit for controlling the energization of said motor, said-first generator having two field windings arranged for differential action relative to each other so as to balance their effects at a given ratio of respective excitation and being provided with at least one other field winding, a second generator having an armature circuit connected with said other field winding of said first generator for controlling its excitation and having at least two regulator field windings, one or said differential generator field windings and one of said regulator field windings being connected with said motor so as to be excited in dependence upon the motor energization, and circuit means under control by the operator for controlling the excitation of the remaining field windings oi said two generators.

WILLIAM HARDING. LAWRENCE G. UPEL, 

